全文获取类型
收费全文 | 4154篇 |
免费 | 115篇 |
国内免费 | 11篇 |
专业分类
化学 | 2321篇 |
晶体学 | 15篇 |
力学 | 130篇 |
数学 | 611篇 |
物理学 | 1203篇 |
出版年
2022年 | 35篇 |
2021年 | 37篇 |
2020年 | 42篇 |
2019年 | 53篇 |
2018年 | 60篇 |
2017年 | 36篇 |
2016年 | 125篇 |
2015年 | 84篇 |
2014年 | 108篇 |
2013年 | 191篇 |
2012年 | 188篇 |
2011年 | 218篇 |
2010年 | 126篇 |
2009年 | 132篇 |
2008年 | 190篇 |
2007年 | 196篇 |
2006年 | 172篇 |
2005年 | 188篇 |
2004年 | 155篇 |
2003年 | 145篇 |
2002年 | 86篇 |
2001年 | 80篇 |
2000年 | 81篇 |
1999年 | 80篇 |
1998年 | 51篇 |
1997年 | 51篇 |
1996年 | 54篇 |
1995年 | 51篇 |
1994年 | 56篇 |
1993年 | 50篇 |
1992年 | 55篇 |
1991年 | 46篇 |
1990年 | 52篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1987年 | 30篇 |
1986年 | 33篇 |
1985年 | 54篇 |
1984年 | 48篇 |
1983年 | 37篇 |
1982年 | 38篇 |
1981年 | 37篇 |
1980年 | 29篇 |
1979年 | 30篇 |
1978年 | 27篇 |
1977年 | 31篇 |
1976年 | 32篇 |
1975年 | 27篇 |
1974年 | 24篇 |
1973年 | 23篇 |
排序方式: 共有4280条查询结果,搜索用时 0 毫秒
21.
Eugenio Hernández Xihua Wang Guido Weiss 《Journal of Fourier Analysis and Applications》1997,3(1):23-41
The main purpose of this paper is to give a procedure to “mollify” the low-pass filters of a large number ofMinimally Supported Frequency (MSF) wavelets so that the smoother functions obtained in this way are also low-pass filters for an MRA. Hence, we are able
to approximate (in the L
2
-norm) MSF wavelets by wavelets with any desired degree of smoothness on the Fourier transform side. Although the MSF wavelets
we consider are bandlimited, this may not be true for their smooth approximations. This phenomena is related to the invariant
cycles under the transformation x ↦2x (mod2π). We also give a characterization of all low-pass filters for MSF wavelets. Throughout the paper new and interesting examples
of wavelets are described. 相似文献
22.
Bernhard Krötz 《Japanese Journal of Mathematics》2007,2(2):303-311
In this paper we raise a question about the boundary of the crown domain of a Riemannian symmetric space X. In case X is of Hermitian type we give an affirmative answer. 相似文献
23.
M. Procop A. Klein I. Rechenberg D. Krüger 《Analytical and bioanalytical chemistry》1997,358(1-2):358-360
An ion beam technique has been developed which allows the preparation of bevels from semiconducting heteroepitaxial structures
with smooth surfaces and very flat angles in the order of 0.1°. The bevels are used for AES depth profiling of heterostructures
by the line scan technique. Measured and calculated line scans from (Al,Ga)As/GaAs and SiGe/Si test structures are compared
to estimate the contributions of the electron escape depth and the ion beam mixing to the depth resolution.
Received: 21 August 1996 / Revised: 14 January 1997 / Accepted: 18 January 1997 相似文献
24.
The near-infrared (NIR) absorption of toluene-sulfonate-diacetylene crystals (PTS) has been investigated during thermal polymerization. It is shown by comparative NIR and IR spectroscopy of deuterated and of protonated PTS at various temperatures, that these absorptions are vibronic in nature. They can be assigned unambiguously to distinct parts of the molecule. No low-lying electronic state has been found. 相似文献
25.
Ohne Zusammenfassung 相似文献
26.
27.
The production cost of cellulolytic enzymes is a major contributor to the high cost of ethanol production from lignocellulosics
using enzymatic hydrolysis. The aim of the present study was to investigate the cellulolytic enzyme production ofTrichoderma reesei Rut C 30, which is known as a good cellulase secreting micro-organism, using willow as the carbon source. The willow, which
is a fast-growing energy crop in Sweden, was impregnated with 1–4% SO2 and steam-pretreated for 5 min at 206°C. The pretreated willow was washed and the wash water, which contains several soluble
sugars from the hemicellulose, was supplemented with fibrous pretreated willow and used for enzyme production. In addition
to sugars, the liquid contains degradation products such as acetic acid, furfural, and 5-hydroxy-methylfurfural, which are
inhibitory for microorganisms. The results showed that 50% of the cellulose can be replaced with sugars from the wash water.
The highest enzyme activity, 1.79 FPU/mL and yield, 133 FPU/g carbohydrate, was obtained at pH 6.0 using 20 g/L carbon source
concentration. At lower pHs, a total lack of growth and enzyme production was observed, which probably could be explained
by furfural inhibition. 相似文献
28.
I. G. Voigt-Martin Z. X. Zhang D. H. Yan A. Yakimanski R. Matschiner P. Krämer C. Glania R Wortmann N. Detzer D. Schollmeyer 《Colloid and polymer science》1997,275(1):18-37
4-Dimethylamino-3-cyanobiphenyl (4-DMA-3-CB) was characterized with respect to linear and nonlinear optical properties in
a crystal as well as in solution. The crystal structure was studied dependent on the crystallization conditions. It is shown
that the crystal structure exhibiting NLO-activity can completely be solved by a combination of electron diffraction and computer
modeling. There are four molecules per unit cell in the space group Pna21 with dimensions a=10.28 Å, b=22.64 Å, c=5.27 Å. From this model structure the values and orientation of the dipole μ and static second order polarizability β can
be calculated. Their relevance to the values obtained by a combination of polarization dependent measurements of Electric
Field Induced Second Harmonic Generation (EFISH) and Hyper-Rayleigh-Scattering (HRS) in solution are discussed. The molecular
second order polarizability tensor was found to be dominated by one single component. The orientations of the dipole and the
vectorial parts of the second order polarizability delivered by the semiempirical calculations are in good agreement with
the results of the EFISH and HRS-measurements and allow a deeper insight into the nonlinear optical properties of the crystal.
Received: 15 April 1996 Accepted: 21 June 1996 相似文献
29.
Zhang JJ Sheng TL Hu SM Xia SQ Leibeling G Meyer F Fu ZY Chen L Fu RB Wu XT 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(16):3963-3969
The first successful attempt to construct 3D supramolecular frameworks with high-nuclear 3d-4f heterometallic clusters as a node is reported. The self-assembly of Ln3+, Cu2+ and amino acid in solution leads to the formation of two polymers, 35-nuclear complex [Sm6Cu29] 1 with a primitive cubic net-like structure and 36-nuclear complex [Nd6Cu30] 2 with a face-centred cubic network type structure. Glycine and L-proline, respectively, were used as ligands. It should be noted that 2 has a chiral framework. X-ray structure analyses show that 1 crystallizes in the triclinic P1 space group (a=19.6451(8), b=20.4682(8), c=20.7046(8) A, alpha=89.453(1), beta=66.290(1), gamma=68.572(1) degrees, V=7003.0(5) A3 and Z=1) and 2 belongs to the cubic P2(1)3 space group (a=b=c=32.4341(3) A, V=34 119.7(5) A3 and Z=4). Both complexes utilize Ln6Cu24 octahedral clusters as nodes and trans-Cu(amino acid)2 groups as bridges. Electrical conductivity measurements reveal that both polymers behave as semiconductors. 相似文献
30.
The binding of a chiral quaternary ammonium ion to a cyclopeptide containing aromatic amino acid subunits is affected not only by the configuration of the cation but also by the configuration of the chiral counterion. Analysis of the binding equilibria shows that complex formation involves interaction of the whole ion pair with the host indicating that steric requirements of the anion influence complex geometry and stability. 相似文献